This invention is in the field of stents for implantation into a vessel of a human body.
Stents are well known medical devices that are used for maintaining the patency of a large variety of lumens of the human body. The most frequent use is for implantation into the coronary vasculature. Stents have been used for this purpose for almost twenty years. Some current stent designs such as the CORDIS BX Velocity(trademark) stent have the required flexibility and radial rigidity to provide excellent clinical results. Yet sometimes such stents are not able to be delivered through extremely torturous or highly calcified vessels.
Many current tubular stents use a multiplicity of circumferential sets of strut members connected by either straight longitudinal connecting links or undulating longitudinal (flexible) connecting links. The circumferential sets of strut members are typically formed from a series of diagonal sections connected to curved sections forming a closed-ring, generally slotted structure. This structure expands radially outwardly as the balloon on which the stent is mounted is inflated to form the element in the stent that provides structural support for the arterial wall.
A closed-cell stent is sometimes considered a stent in which (except at the longitudinal ends of the stent) every curved section of each central circumferential set of strut members has a connection to one end of a flexible link leaving no xe2x80x9cunconnectedxe2x80x9d central curved sections. A stent with at least half of its central (non-end) curved sections being xe2x80x9cunconnectedxe2x80x9d can sometimes be considered as an xe2x80x9copen-cellxe2x80x9d stent. A hybrid design stent is one that has fewer than half of its central curved sections being xe2x80x9cunconnectedxe2x80x9d.
The present invention envisions an improved flexible connecting link used in conjunction with in-phase and half-phase circumferential sets of strut members. The definitions of xe2x80x9cin-phasexe2x80x9d and xe2x80x9chalf-phasexe2x80x9d which describe the orientation of adjacent circumferential sets of strut members will be given in the detailed description of the invention with the aid of several of the figures. By increasing the total length and diagonalness of the undulating connecting links, the present invention is a stent that provides increased flexibility during delivery and enhanced conformability to the shape of a curved artery when the stent is deployed into a curved vessel such as a tortuous coronary artery. By xe2x80x9cincreasing diagonalnessxe2x80x9d is meant that the end points of the flexible connecting links have an increased circumferential displacement each one from the other. That is, more diagonalness means that a line connecting the end points of a flexible links has an increased acute angle relative to a line that lies parallel to the stent""s longitudinal axis.
The BX Velocity stent uses a balloon in which the folds are straight wrapped, to prevent the stent from twisting in a helical manner during deployment. By xe2x80x9cstraight wrappedxe2x80x9d is meant the fold lines of the balloon lie generally parallel to the stent""s longitudinal axis. Such helical twisting can result in significant foreshortening of the stent. The present invention stent system envisions use of a helically wrapped balloon. By xe2x80x9chelically wrappedxe2x80x9d is meant that the folds of the balloon lie at an acute angle relative to a line that is parallel to the stent""s longitudinal axis. When properly oriented relative to the stent, a helically wrapped balloon can cause the stent to lengthen when the balloon is inflated as compared to a foreshortening that can occur when the stent is deployed from a straight wrapped balloon.
Three embodiments of the present invention stent are disclosed herein. Two are closed-cell stent embodiments and one is an open-cell stent embodiment. The first closed-cell stent embodiment uses xe2x80x9cNxe2x80x9d shaped flexible links to connect the ends of the curved sections of adjacent in-phase circumferential sets of strut members. The second closed-cell stent embodiment includes at least one end-to-end spine wherein the diagonal xe2x80x9cNxe2x80x9d flexible links connect from the outside of the curved sections of one circumferential set of strut members to the inside of the curved sections of the adjacent circumferential set of strut members. The spine embodiments also utilize xe2x80x9cin-phasexe2x80x9d circumferential sets of strut members.
The open-cell stent embodiment of the present invention stent uses diagonal xe2x80x9cNxe2x80x9d flexible links to connect adjacent circumferential sets of strut members where only half of the curved sections are connected by a flexible link. The unconnected crowns have shorter diagonal segments so as to reduce the potential for fish-scaling (sometimes called dog-bones) during stent delivery around a bend. xe2x80x9cFish-scalingxe2x80x9d is defined as the tendency of metal struts of a stent to protrude outwardly from the surface of the balloon (like a fish scale) when the pre-deployed stent is advanced through a curved coronary artery.
Although the present invention describes in-phase circumferential sets of strut members where the diagonal flexible links span one-half cycle of circumferential displacement, it is also envisioned that flexible links spanning xe2x85x9 to 1xc2xd cycles are also possible. These configurations of the stents will be described in detail in the detailed description of the invention with the aid of the appropriate drawings.
It is also envisioned that any of the stent designs as taught herein may be used with plastic coatings such as parylene, antithrombogenic coatings such as heparin or phosphorylcholine or anti-proliferative coatings such as paclitaxel or sirolimus.
An additional version of the non-spined, closed-cell embodiment includes two additional configurations. The first of these concepts is a specific technique for widening the diagonal sections within a circumferential set of strut members. It is desirable to taper the diagonal sections to be wider at their center, especially for the end circumferential sets of strut members. Such widening of the diagonal sections of each end circumferential set of strut members will increase the visibility of the stent ends under fluoroscopy. If the diagonal section is widened too close to the point where a curved section connects to a diagonal section of a circumferential set of strut members, this configuration will negatively affect the unbending of the curved section as the stent is deployed. This is a result of creating unwanted plastic strain in the metal if the widened region of the diagonal section is too close to the attachment point of that diagonal section to the curved section. The present invention envisions having a strut segment of uniform width for at least approximately 0.001xe2x80x3 between the end point of the curved section and the start of the widened taper in the diagonal section. A distance of approximately 0.002xe2x80x3 to 0.0003xe2x80x3 is more optimium.
The second of these concepts relates to the longitudinal spacing (i.e., the xe2x80x9cgapxe2x80x9d) between adjacent circumferential sets of strut members. The end structure of a stent is critical to stent deliverability as the leading edge of the stent must bend first as the stent mounted onto the deployment balloon is advanced through a curved artery. Assuming the flexible links for a stent are optimized to be as long and as thin as possible within the gap allowed between adjacent circumferential sets of strut members, the only way to have increased flexibility of the end flexible links is to increase the longitudinal length of the gap between each end circumferential sets of strut members and its adjacent, central circumferential set of strut members. This increased gap will permit a longer (and more flexible) link to connect each one of the two end circumferential sets of strut members to its adjacent central circumferential set of strut members.
Thus it is an object of the present invention to have a stent with circumferential sets of strut members connected each to the other by flexible links wherein a line connecting the flexible link end points that attach to each circumferential set of strut members is diagonally oriented relative to the stent""s longitudinal axis.
Another object of the present invention is to have a closed-cell stent having in-phase circumferential sets of strut members wit h the ends of each diagonal flexible link where they are attached to the circumferential sets of strut members being situated in close proximity to the junction point of a curved section and a diagonal section.
Still another object of the present invention is to have a stent having in-phase circumferential sets of strut members with diagonal flexible links forming an end-to-end spine to prevent stent foreshortening.
Still another object of the present invention is to have an open-cell stent having in-phase circumferential sets of strut members with diagonal flexible links wherein the ends of each diagonal flexible link are connected to the circumferential sets of strut members near the junction of a curved section and a diagonal section.
Still another object of the present invention is to have a closed-cell stent having circumferential sets of strut members with diagonal flexible links wherein the diagonal sections of at least one of the circumferential sets of strut members are tapered to be wider at their center with the taper beginning placed apart from away from the attachment point of the diagonal sections to the curved sections.
Still another object of the invention to have a closed-cell stent with circumferential sets of strut members connected each to the other by flexible links wherein the end diagonal flexible links are longer than the flexible links elsewhere in the stent.
These and other objects and advantages of this invention will become apparent to a person of ordinary skill in this art upon reading of the detailed description of this invention including the associated drawings.